The present invention relates to a state detecting method for detecting the state of an insulation resistance detecting circuit for detecting an insulation resistance between a vehicle body and a DC power source and an insulation resistance fall detector in which the state detecting method is carried out.
Usually, as the above-described insulation resistance fall detector, an earth detecting circuit of an electric motor vehicle, which is disclosed in, for instance, Patent Document 1 is exemplified. The earth detecting circuit serves to detect an earth to a vehicle body E from a battery group B, as shown in FIGS. 5 and 6, in the travel driving circuit system A of the electric motor vehicle comprising the battery group B provided as a high voltage DC power source (for instance, 200 to 300 V), an inverter 2 as a DC-AC converter for converting a direct current fed through a plus bus bar 4 as an anode DC feeder and a minus bus bar 5 as a cathode DC feeder from the battery group B to an alternating current, and an alternating current electric motor (an AC electric motor) 3 to which the alternating current is fed through a U phase line 6, a V phase line 7 and a W phase line 8 as the alternating current (AC) feeders from the inverter 2. The ground detecting circuit includes an oscillation circuit 10 as an alternating current (AC) signal output circuit and a detecting part 20 as a voltage level change detecting circuit. A connecting point P of the oscillation circuit 10 and the detecting part 20 is connected to the plus bus bar 4 of the battery group B of the travel driving circuit system A by a coupling condenser 10A to interrupt a direct current (DC) component.
In the oscillation circuit 10, an oscillator 11 generates a rectangular wave pulse of a predetermined frequency with a duty ratio of 50% (see the right and left parts of FIG. 6A). An impedance converter 12 of a next stage outputs the rectangular wave pulse of the oscillator 11 with the duty ratio as it is. An alternating current (AC) signal output of the oscillation circuit 10 appears in the connecting point P through a detection resistance 13. The detection resistance 13 serves as a potential divider together with an earth resistance 31 upon generation of the earth.
The detecting part 20 is provided with a comparator 21 for comparing the voltage level of the connecting point P of the detection resistance 13 and the coupling condenser 10A at which the AC signal output of the oscillation circuit 10 appears with reference voltage V1. The connecting point P is connected to an inverse input terminal of the comparator 21. To a non-inverse input terminal of the comparator 21, a reference voltage circuit to which the reference voltage V1 is set by potential dividing resistances 22 and 23 is connected.
Protecting diodes 15 to 18 are connected to the output side of the impedance converter 12 and the input side of the comparator 21 to protect an operational amplifier forming the impedance converter 12 and the comparator 21 from reverse voltage and over-voltage upon generation of the earth.
According to the above-described circuit structure, when the earth is not ordinarily generated, the insulation resistance 31 can be considered to be infinite. From the connecting point P at which a value obtained by dividing the potential of the rectangular wave pulse by the insulation resistance 31 and the detection resistance 13 appears, a rectangular wave pulse of a peak value higher than a preset reference voltage V1 is outputted (see the left side of FIG. 6D).
Thus, to the inverse input terminal of the comparator 21, the rectangular wave pulse having the peak value higher then the preset reference voltage V1 is inputted. The output of the comparator 21 is the rectangular wave pulse having a duty ratio of 50% (see the left side of FIG. 6C). Then, smooth voltage Vr obtained by a smoothing circuit 26 having a resistance 24 and a condenser 25 is lower than the reference voltage (see the left side of FIG. 6D). The smooth voltage is inputted to the non-inverse input terminal of a comparator 27 and the output of the comparator 27 becomes a low level showing a normal state (see the left side of FIG. 6E).
However, when the earth is generated between the minus bus bar and the vehicle body E to lower the insulation resistance 31 shown in FIG. 5, from the connecting point P at which the value obtained by dividing the potential of the rectangular wave pulse by the insulation resistance 31 and the detection resistance 13 appears, a rectangular wave pulse of a peak value lower than the reference voltage V1 is outputted (see the right side of FIG. 6(b)). Thus, to the inverse input terminal of the comparator 21, the rectangular wave pulse having the peak value lower then the reference voltage V1 is inputted. The duty ratio of the comparator 21 is changed to 100% (see the right side of FIG. 6C)
As a result, the smooth voltage Vr obtained by the smoothing circuit 26 having the resistance 24 and the condenser 25 is higher than the reference voltage (see the right side of FIG. 6D). The smooth voltage is inputted to the non-inverse input terminal of the comparator 27 and the output of the comparator 27 becomes a high level showing the fall of the insulation resistance (see the right side of FIG. 6E). As described above, when the earth is generated in the battery group B, the earth can be detected from the logical level of the comparator 27. Patent Document 1: JP-A-8-70503
However, in the above-described earth detecting circuit, whether the earth detecting circuit itself is in a failed state or a normal state can not be disadvantageously detected.